Energy Expenditure
Eckert & Randall Chapter 16

I. Introduction

A. Animals catabolize, but are not 100% efficient

1. "Waste" energy given off as heat - can be used to warm reactions to more efficient temperature
2. Heat may be a problem in warm climates

II. Concept of Energy Metabolism

A. Anabolism = simple chemicals united into larger chemicals - requires energy

1. Measured as growth, increase in dry weight, increase in nitrogen in protein, etc.

B. Catabolism = Complex chemicals broken down to simple chemicals - release energy

C. Metabolism = Anabolism + catabolism

1. Measured by heat production (calories)
2. Influenced by temperature, hormone levels, osmotic stress, feeding, movement, age, size, other stresses, etc.

III. Temperature Classification of Animals

A. Poikilothermic = body temperature varies with environmental temperature

1. Often is used to designate animals of varying body temperature also.
2. Many deep sea fish are poikilothermic and have very stable body temperartures
3. Many poikilotherms as lizards may behaviorally regulate body temperature

B. Homeothermic = regulate their body temperature close to a set point by producing heat when needed or cooling when needed

1. Some of these animals may become poikilothermic overnight like humming-birds
2. Some animals keep a rather constant body temperature when active or rapidly growing, but not when inactive or if the environment gets too cool.

C. Endothermic = use metabolic energy to warm and cool the body

1. Mammals and birds, some fish as tuna, a few flying insects
2. Usually insulated with fur, feathers, or fat

D. Ectotherms = animals that use environmental heat sources to warm the body more than metabolic enrgy if the animals regulate body temperature at all.

IV. Measurement of Metabolic Rates

A. Basic Metabolic Rate = the steady metabolic rate under conditions of minimal environmental stress, post digestive and post absorptive in homeothermic animals
B. Standard Metabolic Rate = the metabolic rate at rest and post absorptive at a given environmental temperature for poikilothermic animals
C. Oxygen debt = the amount of energy that is not immediately measured by oxygen consumption when an animal exercises, but is measureable after exercise stops. Fig 16-2 p670.

1. Oxygen debt accrues in the form of lactic acid build up which must be metabolized further via mitochondria to CO2, H2O and some back to glucose.

D. How could you measure energy uptake?

1. Burn dried food sample in a bomb calorimeter. Measure the heat given off.
2. Burn the waste produced by that food in urine and feces and measure the heat given off.
a. Does the body burn food the same way a flame does?

(1) Gut bacteria
(2) Not all enzymes are present to use all food - should appear in feces.
(3) Bacterial metabolism is used for heat production and bacterial bodies appear in feces.

E. Direct Calorimetry -

1. Can burn food, waste, tissue produced, etc. by burning.
2. Can measure the heat given off by an animal.

F. Indirect Calorimetry

1. Measuring O2 comumption & CO2 production

a. The amount of heat produced in metabolism is related to oxygen consumption. Table 16-1 p 673.
b. Oxygen concentrations can be measured by respirometers like the Gilson that uses pressure and volume changes or like the oxygen analyzer that uses an electro-chemical cell. Carbon dioxide can be measured by infrared absorption.
c. Assumptions when converting oxygen consumption data to heat values:

(1) All reactions are aerobic. Usually true at rest.
(2) That heat equivalents are the same for all substrates. Not true and can introduce errors around 10%. Respiratory quotients can help reduce the error.

2. Respiratory quotient

a. Notice the RQ and related data in Table 16-1 p 673

(1) For carbohydrates the RQ is 1.00
(2) For fats RQ is 0.71
(3) Proteins produce an RQ of around 0.80 when converted to urea. The amount of protein used can be estimated by measuring the amount of urea nitrogen.

b. Aerobic animals at rest and post-absorptive and not starving burn very little protein and under these conditions it may be ignored.
c. Assumptions of RQ are:

(1) CO2 gas released is the same amount as produced in the tissues over the time period measured. Need a steady state

G. Specific Dynamic Action - Fig 16.5 p 675

1. The requirements of post-absorptive are because of an increase in heat production due to the absorption and digesion of food. The amount of heat produced also depends upon the substrate. This increased heat production can last for hours in birds and ammals and days in ectotherms.

a. Carbohydrates and fats increase heat production by 5-10% of their total calories
b. Protein increases heat production by 25-30% of total calories consummed.

H. Energy Storage

1. Proteins are not stored for energy use.
2. Fat stores 9.56 kcal/g and have no water of hydration
3. Carbohydrates yield 4.1 kcal/g plus their are 4-5g of water/ g carbohydrate. This reduces the value to 0.8 kcal/g hydrated carbohydrate. Only carbohydrates and protein can produce glucose however and this is required by some tissues such as nervous tissue. Also when using energy water is useful as well, to replace what is lost.

V. Body size and Metabolic Rate

A. Retaining body proportions, doubling the height of an animal increases the surface area 4X, cross-sectional area 4X, and mass 8X. What would this do to metabolic rate?
B. The metabolic rate increases at an exponential rate of 0.75 with mass. See Figures 16-7 p 677. These are the so-called mouse to elephant curves.
C. The body surface area of animals from mouse to whale tends to increase at an exponential rate of 0.63. Fig 16-9 p 678
D. The metabolic rate increase vs mass in adult animals of the same species is exponential at 0.67. Fig 16-10
E. Smaller animals have increased mass specific metabolic rates. Smaller animals have denser capillaries, more mitochondria per cell, more electron transport molecules per cell, and higher levels of carbonic anhydrase in their RBC's.

VI. Effects of Temperature on Animals

A. Q10 is the rate of metabolic increase over a 10 degree temperature span

1. It is empirical - must be measured (can not be predicted) for every given reaction or organism.
2. Usually between 2 & 3 for chemical reactions.; about 1 for diffusion & physical processes; and it widely varies in animals of different species. It is strictly something to be measured.

VI. Determinants of Body Heat & Temperature

A. Heat is the heat energy in the animal. It consists of the temperature times the heat capacity per gram of tissue. There is so much water in animals that the heat capacity is near 1 cal/g tissue. The total heat in an animal them depends greatly on size
B. Obvious formula: body heat = heat produced + heat gained - heat lost. Wow!
C. Heat transfer depends upon

1. Surface area. As animals get larger the surface area/g tissue decreases. Temperature change is thus more difficult.
2. Temperature gradient between surface temperature and ambient temperature.
3. Specific heat conductance at surface. Modified by insulation and environment as water or air.

D. Regulation of heat transfer

1. Behavioral - as orientation to sun or ground surface, moving to warmer or cooler spot etc.
2. Autonomic - as sweating or salivating for evaporative cooling or increased insulation by trapping more air in feathers or hair - piloerector muscles.
3. Adaptive - as thicker hair, more fat, - usually hormonally controlled.

VII. Temperature Relations of Ectotherms

A. Ectotherms in cold environments

1. Several types of animals (Chironomides & Barnacles) can withstand freezing temperatures by allowing interstitial fluid to freeze (nucleating promoters) and concentrating solutes in intracellular fluid.
2. There are fish that live at sea temperatures below freezing for its body fluids. Their body water contains no nucleating substances and it is "super-cooled". Bumping it can cause instant crystal formation & death.
3. Some animals (insects, an ice fish in antarctica) contain anti-freezes in their body fluids (glycerol, glycoprotein) . The mechanism of the glycerol is strictly molar, the mechanism of the glycoprotein is unknown.

B. Ectotherms in hot environments

1. Behavioral exposure to sun or shade is common
2. Marine Iguana can divert blood from skin when diving.
3. Dessert Iguana can still have 1/2 of Hb oxygenated at 50o and survives.

C. Advantages & disadvantages to ectothermy

1. Possibility of smaller body size in some environments
2. Lower metabolic rates, less food needed
3. less water needed
4. Less able to sustain high activity level

VIII. Temperature strategies of Heterotherms

A. Flying insects use muscle activity to warm , shivering thermogenesis
B. Some insects can regulate blood flow
C. Bees may move in and out of the swarm
D. Tuna continually swim and have a rete mirabile for blood to skin, keeping core temperatures high (+10^oC)

IX. Temperature relations of Endotherms

A. Thermal Neutral Zone

1. This is the range of temperatures over which the animal does not expend extra energy to either cool or warm itself.
2. It is bounded by the upper and lower critical temperatures

B. Thermogenesis

1. Shivering thermogenesis - controlled by the brain, causes muscular contractions to create heat.
2. Non-shivering thermogenesis - some animals can burn fat over the body with little ATP produced. The energy of catabolism is used for the heat instead.

a. Brown Fat - A number of animals (baby humans, squirrels, bats,etc) have specialized fat pads around the neck and shoulder blades that consists of fat cells with large numbers of mitochondria. These generally heat up the vertebral artery to the brain. The fat is supplied with nerves from the sympathetic nervous system.

C. Endothermy in Cold Environments

1. Large volume to surface area - lower thermal conductance
2. Terrestrial animals trap air in pelage
3. Marine mammals use blubber - fat does not conduct heat well and even small seals may have a layer of a foot of fat around them
4. Counter-current blood flow in seals, birds, elk, moose, etc
5. Choice of veins returning blood to heart. Internal veins retain heat.
6. Low temperature lipids in cell membranes of cold adapted tissues

 

D. Endothermy in hot environments
1. Large size can reduce surface for absorption, increase heat sink so that there is less of a temperature rise with heat absorbed

a. Conservation of water in respiration via narrow nasal channels (camel, kangaroo rat)

2. Pelage can reduce absorption of heat.
3. Evaporative cooling - 585 cal/g

a. Sweating - great if water is easily available

(1) Pigs desire mud because they have few sweat glands

b. Panting. Breath in nose and out mouth when panting

(1) Much of this is dead space air movement to avoid loss of too much CO2.

X. Hypothalamic control of body temperature

A. Fevers & aspirin

XI. Dormancy

A. Sleep

1. Body temperature drops some - hypothalamus less sensitive
2. May be due to accumulation of interstitial substances building up during wakefulness.

B. Torpor

1. A drop in temperature and metabolic rate in small mammals and birds. Necessary to keep from starving.

C. Hibernation

1. Body temperatures may drop 20o C or more.
2. Animals arouse occasionally.
3. Dangerously low ambient temperatures will arouse the animal.
4. Blood flow about 10% of normal.

D. Estivation

1. A "summer sleep" to avoid heat and desiccation.
2. Snails, Australian crocodiles, lung fish

XII. Energetics of locomotion.

A. Just study the chart 16-20 p 686.